Early life experience, which is crucial for later development, is shaped substantially by maternal behavior (MB), which in turn is subject to several endocrine factors. One factor is oxytocin (OXY): we have discovered and others have replicated that central administration of OXY induces the rapid onset of MB. Central administration of OXY antiserum or an antagonist blocks the onset of MB in ovarian steroid-treated and parturient rats. Neurons in the basal forebrain that synthesize OXY project to numerous brain sites that are involved in the regulation of MB. We have observed dramatic prepartum and postpartum changes in the intensity and topography of CNS OXY immunostaining as well as changes in immunoreactive OXY content in specific brain sites. To extend our understanding of changes in the activity of CNS OXY systems during late pregnancy, parturition and early lactation we will 1) measure immunoreactive OXY content and binding in all brain sites that are critical to MB and receive OXY projections, 2) locate and quantify OXY mRNA (an indirect measure of OXY synthesis) using in situ hybridization and 3) assay CSF OXY concentration as a measure of total central release at several prepartum and postpartum time points (Study I). The rise in estrogen and fall in progesterone in late pregnancy are essential for parturition, lactation and the onset of MB in the rat. We will test (Study II) whether each of these ovarian steroid shifts is necessary for changes in CNS OXY dynamics (regional brain immunoreactive content, binding, immunohistochemistry, in situ hybridization, CSF concentration). Postpartum changes in OXY immunostaining in the forebrain persist during lactation but revert to the pregestational pattern by five days after separation of pups at weaning. We will, therefore, test the necessity of interaction with pups for the development and persistence of postpartum changes in CNS OXY dynamics (Study III). We hypothesize that the onset of MB involves changes in specific OXY dynamics in discrete brain sites. To test, this hypothesis we will 1) locate and quantify changes in CNS OXY dynamics occurring concomitant with the onset of MB and 2) compare CNS OXY dynamics between two ovarian steroid treatments; one that does and one that does not stimulate rapid onset MB (Study IV).